All filters are parfocal Astrodon: most frustration nell'astrofotografia with CCD is that of having to find the correct firing position with the filter H alpha: the narrow band filters keep out much light so you must use a very bright star to perfect focus. All filters Astrodon Tru-Balance L, C, R, G, B, H-Alfa, OIII and SII are parfocal on optics with focal ratio less than f / 4. Login This means you can adjust the focus by using a filter C or G, install a filter H-Alpha and immediately proceed with the acquisition. This is one of the key advantages of the Tru-Balance filters Astrodon that made them cosŕ¬ popular in a short time. The thickness of the filter determines how it can be parafocality: Tru-Balance filters are dichroic filters with very resistant treatments deposited on a single glass substrate highquality. The reduced thickness (3 mm) of this substrate is controlled accurately with error +/- 0.05mm. The change from lot to lot is less than 0.1mm, measuring slightly less than the critical areas of focus of a system f / 5. On request it is possible to select within various batches filters with less variation and then get a set of filters that have the exact same thickness (prices on request).